U.S. patent number 3,944,637 [Application Number 05/441,645] was granted by the patent office on 1976-03-16 for increasing the yield of cast plastic lenses.
This patent grant is currently assigned to Buckbee-Mears Company. Invention is credited to Herbert M. Bond, Daniel L. Torgersen.
United States Patent |
3,944,637 |
Bond , et al. |
March 16, 1976 |
Increasing the yield of cast plastic lenses
Abstract
A terpolymer plastic lens having equal or greater light
transmission, greater hardness and scratch resistance equal to the
monomers used in prior art plastic lenses is obtained by the
co-polymerization of diethylene glycol bis (allyl carbonate),
methyl methacrylate and methacrylic acid or acrylic acid.
Inventors: |
Bond; Herbert M. (Stillwater,
MN), Torgersen; Daniel L. (St. Paul, MN) |
Assignee: |
Buckbee-Mears Company (St.
Paul, MN)
|
Family
ID: |
23753719 |
Appl.
No.: |
05/441,645 |
Filed: |
February 11, 1974 |
Current U.S.
Class: |
264/1.1;
351/159.01; 264/2.3; 264/331.18; 264/300; 526/318.4 |
Current CPC
Class: |
B29D
11/00009 (20130101); C08F 18/08 (20130101); G02B
1/041 (20130101); G02B 1/041 (20130101); C08L
31/00 (20130101); G02B 1/041 (20130101); C08L
33/12 (20130101) |
Current International
Class: |
B29D
11/00 (20060101); C08F 18/08 (20060101); C08F
18/00 (20060101); G02B 1/04 (20060101); B29D
011/00 () |
Field of
Search: |
;264/1,300,331
;260/80.81,29.6TA,80.8,77.5D ;351/160 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: White; Robert F.
Assistant Examiner: Auville; Gene
Attorney, Agent or Firm: Jacobson and Johnson
Claims
We claim:
1. In the process of preparing a polymerized lens in a glass mold
by preparing a reactant mixture of methyl methacrylate, diethylene
glycol bis (allyl carbonate) and an acid selected from the group
consisting of acrylic acid and methacrylic acid, which method
comprises: preparing a monomer mixture of methyl methacrylate and
diethylene glycol bis (allyl carbonate) wherein the methyl
methacrylate monomer ranges from about 10 to about 50% by weight of
the monomer mixture, balance diethylene glycol bis (allyl
carbonate); adding to said monomer mixture a lubricant and a
catalyst of benzoyl peroxide ranging from between about 1 to 1.5%
by weight of the monomer mixture; exposing said reactant mixture to
a thermal treating zone to polymerize said reactant mixture and
form a terpolymer therefrom with the improvement comprising adding
to said monomer mixture an acid selected from the group consisting
of acrylic acid and methacrylic acid, with said acid ranging from
between about 1/2 to 5% by weight of said monomer mixutre.
2. The process as set forth in claim 1 being particularly
characterized in that a lubricant is added to said reactant mixture
prior to polymerization thereof, and wherein said lubricant is
selected from the group consisting of mono and dialkyl phosphates
having from between about 16 and 18 carbon atoms in the alkyl
radical, and stearic acid, said lubricant being present in said
reactant mixture in a quantity of less than about 20 ppm.
3. The process as set forth in claim 1 wherein said treating zone
increases the temperature of said reactant mixture slowly until a
temperature of about 100.degree.C. is reached.
4. The process as set forth in claim 1 being particularly
characterized in that said acid is methacrylic acid, and is present
in said reactant mixture in an amount greater than 1/2 % by weight
of the reactant mixture.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to a cross-linkable terpolymer
that is polymerized and solidified by heat curing and, more
specifically, to a terpolymer suitable for use as lenses in eye
glasses.
DESCRIPTION OF THE PRIOR ART
Plastic lenses are old in the art and conventionally have been made
from polycarbonate resins such as diethylene glycol bis (allyl
carbonate) which is known commercially in its monomeric form as
allyl diglycol carbonate and in either its monomeric or polymerized
form as CR-39. It therefore should be understood that the use of
any of these terms are commonly accepted identifying the same
monomer by those in the art.
Because of the clarity, high strength and high impact resistance of
CR-39, it is suitable for use in plastic lenses for eye glasses.
The CR-39 is not only suitable for lenses for eye glasses but is
widely preferred and virtually exclusively used because of its high
resistance to discolorization and its resistance to warping or
distortion. A more complete description of this type of monomer and
the manufacture of this type of plastic lenses can be found in the
prior art Beattle U.S. Pat. No. 2,542,386. Still another material
used for lenses for eye glasses is described in the prior art
Emerson et al U.S. Pat. No. 3,297,422 which suggests the use of
methyl methacrylate monomer. Still other copolymers of methyl
methacrylate and CR-39 are known.
The present invention relates to improvements in plastic lenses by
the discovery that a terpolymer of allyl diglycol carbonate, a
saturated alkyl ester of methacrylic acid such alkyl group having
from between one and twelve carbon atoms in the chain, and an acid
selected from the group consisting of methacrylic acid and acrylic
acid produce polymers useful as lenses which are capable of being
mass produced as well as having improved characteristics over the
prior art monomer and co-polymer lenses.
While the manufacture of formulation of a composition of a monomer
for preparing a lens is well known, the process for mass production
of a lens comprised of a polymer consisting of two or more monomers
has been extremely difficult due to the problem of erratic release
of the plastic lens from the glass mold. The problem of release
involves the control of the adherence of the surface of the plastic
lens to the surface of the mold. To obtain high yields, the
adherence of the surface of the plastic lens to the surface of the
glass mold must be sufficient to hold the plastic lens to the glass
mold through the curing stage, but weak enough to allow easy
separating after curing.
In order to manufacture plastic lenses for optical use, one usually
casts the plastic lens in a glass mold which has the desired
curvature therein. One of the problems with the use of two or more
monomers to form a plastic lens is that it has been extremely
difficult to control the release of the lens from the glass mold.
One prior art method used to control the release is the use of
surface lubricants such as stearic acid or Ortholeum 162
(tradename). However, to date, the known lubricants of this type
have not provided the consistent release of the lenses from the
mold and consequently the yield has been poor. That is, there is a
tendency of the lens to release erratically from the mold by either
releasing from the mold too soon or not releasing at all. If the
lens releases too soon, i.e., before the curing is completed, the
lens may crack thus rendering the lens unsuitable for optical
purposes. On the other hand, if the lens does not release easily
after curing, one can ruin both the plastic lens and the glass mold
by attempting to physically separate the plastic lens from the
glass mold. In either case, the lens may be ruined and rendered
unsuitable for use as a plastic lens for eye glasses. The strange
part of this phenomena of release is that the use of additives
which are known to act as lubricants by decreasing the adherence of
the surface of the glass mold to the surface of the plastic do not
yield consistent results. That is, the amount of lubricant can be
held constant with identical polymers and in one case the lens may
not release from the mold and in the other case the lens may
release from the mold prior to completion of the curing. Thus,
while the lens is of optical quality without this third monomer,
one cannot obtain high consistent yields unless the third monomer
is used.
The present invention is the discovery that the addition of acrylic
acid or methacrylic acid in minor amounts produces a terpolymer
that still has the desired optical qualities for use in eye glasses
yet provides high yields because the lens consistently releases
from the surface of the glass mold at the proper time or can easily
be forced to release. The consistent release is somewhat surprising
in that the acrylic and methacrylic acid are believed to increase
the adherence of the surface of the glass mold to the surface of
the plastic lens. It will be recalled that one aspect of the
problems was the tendency of the surface of the glass mold to
adhere too strongly to the surface of the plastic lens.
Nevertheless, the addition of the third monomer in the polymeric
mass has been found to solve both the problem of pre-release or
over-adherence of the lens to the mold. A further advantage is that
the use of methacrylic acid as a third monomer has been found to
increase the hardness of the lens without having any adverse
effects on the other optical properties of the lens such as
abrasion resistance and optical clarity.
Summary of the Invention
Briefly, the invention comprises the discovery that one can mass
produce plastic lenses with high yields without sacrificing the
optical characteristics of the lens.
In another feature, the invention comprises the discovery that the
addition of 1/2 to 5% by weight of an acid selected from the group
consisting of acrylic acid and methacrylic acid to a reactant
mixture lens comprised of allyl diglycol carbonate and methyl
methacrylate consistently yields lenses of optical quality.
Description of the Preferred Process
While increasing the concentration of methyl methacrylate produces
improvements in certain physical parameters, notably impact
resistance, tensile strength and hardness, other factors which are
not readily measurable have demonstrated that if the methyl
methacrylate is about 10 to 12% by weight and the allyl diglycol
carbonate comprises the balance of about 90 to 88% by weight, we
obtain a lens of superior optical quality if we maintain the
concentration of the copolymers within this range. The basis for
the above percentage being suitable for plastic lenses is based
primarily on visual inspection and interpretation of abrasion
tests. That is, there are various types of abrasion resistance
tests that can be performed on the plastic lens, however, none of
them are very well suited for measuring the scratch or abrasion
resistance of a lens under actual use. It has been found that one
of the most simple tests, namely, lightly rubbing a pencil eraser
over the surface of the lens a predetermined number of times
produces the best indication of the scratch resistance of a lens in
actual use. While no absolute measurements of the abrasion
resistance are obtained, one can make a fairly accurate visual
comparison between different lenses. It is this analysis that has
led to the determination that a lens having about 10 to 12% methyl
methacrylate and balance of about 90 to 88% by weight of diethylene
glycol bis (allyl carbonate) is the most preferred polymer for use
in eye glasses because of the excellent optical characteristics.
Nevertheless, a plastic lens having as low as 0% methyl
methacrylate to as high as 50% methyl methacrylate has optical
characteristics which allow it to be used as a plastic lens.
The heart of the present invention involves the discovery that
adding a third monomer selected from the group consisting of
acrylic acid and methacrylic acid to form a terpolymer with allyl
diglycol carbonate and methyl methacrylate does not have a
detrimental effect on the optical characteristics of the plastic
lens yet it eliminates the problems of erratic release of the lens
from the mold to allow one to mass produce the plastic lenses.
In order to more fully describe the process, the following examples
are included herewith.
Typically, the lenses are cast in blanks which are approximately
1/4 inch thick and have a diameter of about 21/2 inches with one
surface convex and the other surface concave. This particular shape
is required so that the lens blank can be ground to the proper
prescription. The casing of this shape lens with monomers such as
CR-39 is shown and described in the prior art Beattle U.S Pat. No.
2,542,386.
EXAMPLE 1
In order to manufacture a lens suitable for use in an eye glass,
one prepares and mixes a solution of 12 parts by weight of methyl
methacrylate containing 1.25% by weight of benzoyl peroxide and
0.15% by weight of a UV absorber such as p-methoxybenzylidene
malonic acid dimethyl ester. After the solution has been mixed it
is dried by placing about 50 grams of anhydrous sodium sulfate per
1000 ml of solution. Typically, a minimum of 50 grams of anhydrous
sodium sulfate per 500 ml of solution is sufficient to remove any
water which may be in the solution. Next, one vacuum filters the
solution through filter paper to remove the sodium sulfate. Next,
10 parts per million (by weight) of a lubricant such as Ortholeum
162 are stirred into the solution. At this point a reactant mixture
is prepared containing the 12 parts by weight of the solution of
methyl methacrylate, 88 parts by weight of diethylene glycol bis
(allyl carbonate) and with 1/2% of acrylic acid being stirred into
the solution. The solution and the container are then placed in a
vacuum chamber where the air is removed. This serves to remove the
air bubbles in the solution thus eliminating the possibility of the
plastic lens having internal air bubbles. Next, the evacuated
container is pressurized with nitrogen at about 15 psi. After
pressurizing the container with nitrogen the solution is then
forced under pressure into the molds. Once the solution is in the
molds the mold is placed in an oven at 65.degree. C. for a minimum
of about 3 hours followed by raising the temperature from
65.degree.-75.degree. C. in a minimum of about 4 hours which is
followed by raising the temperature from 75.degree. C. to
100.degree. C. in a minimum of about 10 hours, whereupon the molds
are removed from the oven and allowed to cool at room
temperature.
EXAMPLE 2
Additional lenses were cast according to the process of Example 1
except that the amounts of acrylic acid present were increased up
to 5.0% by weight by using .5%, 1%, 1.5%, 1.65%, 1.75%, 1.85%,
2.0%, 3.0% and 5.0% acrylic acid. In all cases the plastic lens
exhibited optical characteristics as good or better than copolymers
and without any problems of pre-release from the mold or adherence
of the lens to the mold except in the case of 5.0% acrylic acid
which adhered very strongly to the mold.
EXAMPLE 3
Additional lenses of Example 2 were repeated with the exception
that methacrylic acid was utilized in lieu of the acrylic acid. The
amounts of methacrylic acid were .5%, 1%, 1.5%, 1.65%, 1.75%,
1.85%, 2.0%, 3.0% and 5.0%. The lenses all exhibited excellent
optical characteristics without pre-release from the mold or
adherence to the mold except in the case of 5.0% methacrylic acid
which was slightly yellow and released with difficulty. In
addition, the lens containing methacrylic acid in the polymer
generally had a hardness of 12.5 to 13 based on a diamond pyramid
hardness using 500 grams with a 136.degree. diamond head.
Approximately 500 lenses were cast according to Example 1, Example
2 and Example 3 without a single failure due to pre-release or over
adherence of the lens to the mold.
EXAMPLE 4
Additional lenses were cast according to the process of Examples 1
and 2 except that the methyl methacrylate component in the methyl
methacrylate allyl diglycol carbonate portion of the terpolymer
ranged from 10 to 50% in 10% graduations and the allyl diglycol
carbonate balance ranged from 90 to 50%. No adverse effect of
optical characteristics as a result of the acrylic or methacrylic
acid were observed yet the yield was in excess of 99%.
In order to obtain a clean release of the lens from the mold,
initially only a lubricant was added to the mixture. Typical well
known lubricants for use in such an application are the mono alkyl
phosphates and the dialkyl phosphates or the mixtures thereof which
are sold under the tradenames Ortholeum 162 or Zelec U.N. Another
suitable lubricant is not necessary to the invention to enhance the
optical characteristics of the lens for eye glasses but merely aids
in stripping the lens from the mold. In fact, impurities in the
mixutre such as dirt or dust which may be present in the solution
can also act as a lubricant to assist in release of the lens from
the mold, however, these types of lubricants are generally not
desirable because of the residue that remains in the lens, however,
the lubricant alone has not been sufficient to produce high
yields.
Further tests were also conducted in which the lubricant was varied
from 0 parts per million to 25 parts per million. With less than 6
parts per million of lubricant there was a greater tendency of the
polymerized lens to adhere to the mold causing difficulty in
removing the mold from the polymerized lens and with lubricants
less than 2 parts per million the number of pullouts greatly
increased. However, the lens itself was still of optical quality if
the mold could be carefully stripped from the lens. While it has
been found that no particular lubricant produces better release
from the mold, it has been found that using a lubricant in excess
of 20 parts per million may cause pre-release of the lens from the
mold which renders the polymerized lens unsuitable for optical
use.
Even after the use of lubricants in the preferred range, it was
found that the yield of usable lenses was less than acceptable
because of lack of consistency. That is, the cost per usable lens
was sufficiently high so as to render the process and lens too
costly except for custom use. At this point, we discovered that we
could increase the yield of the lens by adding a third monomer to
the mixture. The benefits from the addition of the third monomer
were threefold; the first being there was no adverse effect on the
optical characteristics of the lens such as clarity, hardness or
abrasion resistance; the second benefit was that the hardness
actually increased about one unit with the third monomer; and the
third benefit was that the yield of usable lens was in excess of
99% as opposed to about 50% without the third monomer.
Generally, to manufacture our improved lenses the polymerization
mixture is prepared by mixing the allyl diglycol carbonate monomer
with the methacrylate ester and a catalyzer such as benzoyl
peroxide, acetyl peroxide or isopropyl percarbonate. However, the
benzoyl peroxide catalyst is preferred because it produces a
substantially harder lens when used with the preferred ratio of the
two monomers. More specifically, a suitable range of benzoyl
peroxide is about 1.0 to 1.5% by weight with 1.25% by weight of the
total polymer mixture being optimum. While we have described our
preferred amount of catalyst, it should be understood that more or
less amounts of catalyst will work but that the terpolymer does not
consistently meet the characteristics required for clear, hard,
plastic lenses if the amount of catalyst is increased or decreased
beyond the preferred ranges.
Generally, all plastics are damaged in one way or another by
degradation due to the ultra violet portion of the light radiation
from the sun. However, our polymer mixture is suitable for use in
eye glasses without any UV absorbers but in the event further long
term protection against degradation is desired, UV absorbers are
available for incorporation in the reactant mixture. Typically, the
UV absorbers are added in small concentration of less than one-half
percent. Examples of some well known compounds used as UV absorbers
are benzophones, benzotriazoles, substituted acrylonitriles and
phenol-nickel complexes. In the present invention it is preferred
to use p-methoxybenzylidene malonic acid dimethyl ester as a UV
absorber preferably in an amount of about 0.2%.
EXAMPLE 5
Further tests were conducted in accordance with the procedure of
Example 1 in which the only components of the mixture were the
diethylene glycol bis (allyl carbonate) O[CH.sub.2 CH.sub.2
OCOO(C.sub.3 H.sub.5)].sub.2, methyl methacrylate CH.sub.2 :
C(CH.sub.3)COOCH.sub.3 an acid selected from the group consisting
of acrylic acid and methacrylic acid, and benzoyl peroxide (C.sub.6
H.sub.5 CO).sub.2 O.sub.2. In these tests the amounts of diethylene
glycol bis (allyl carbonate) present in the diethylene glycol bis
(allyl carbonate) methyl methacrylate portion were varied from 50
to 90% in 10% increments and the methyl methacrylate balance was
varied from 10 to 50% with the catalyst benzoyl peroxide being
varied from 1.0 to 1.5% by weight of the mixture of methyl
methacrylate and diethylene glycol bis (allyl carbonate). The
acrylic acid and methacrylic acid ranged from 1/2 to 5% by weight
of the mixture. The lenses produced were of the same optical
quality as those with either the lubricant or the lubricant and the
UV absorber. However, the yield of lenses without the lubricant was
not equal to the yield with lubricants.
* * * * *